In communications applications, message information is modulated onto the transmitted signal and a receiver system must demodulate (detect and extract) the embedded message information. In active imaging applications, imaging information regarding the environment is embedded onto the modulated transmit signal and a receiver system must demodulate (detect, extract and estimate) the embedded environmental imaging information. The transmitted signals in these applications may undergo many transformations as they propagate and before they are received. More specifically, phenomenon, such as multipaths, clutter, dispersion, motion, jamming, intercepts, and/or spoofing, may degrade the transmitted signal prior to reception by the receiver. Further, cost-performance tradeoffs may result in additional degradation of the signal at the transmitter and/or receiver, i.e. the need to use simple and less expensive transmitter and receiver apparatuses to keep costs down often cause further degradations in signal quality. Accordingly, there is a need for apparatuses and/or methods that efficiently and effectively modulate/embed information and efficiently and effectively demodulate (detect and extract) the embedded information while rejecting undesired information.
More specifically, the desirable features of a modulated signal in such an apparatus and/or method should include: stealth/security; utilization of all available degrees of freedom in space (aperture), time (duration) and frequency (bandwidth); low cost implementation; and immunity/robustness to degradations due to low cost implementations (non-linearities, dispersion), channel/environmental degradations (multipaths and angular/time/frequency dispersion), relative motion, antenna/transducer mismatch and imperfections, and intentional denial (countermeasures: jamming, spoofing, repeating).
Additionally, the demodulator should be able to efficiently detect and extract the embedded information with a low cost implementation by utilizing all available degrees of freedom and coherently combining desired energy while simultaneously minimizing undesired energy in the received signal. Further, the overall information extraction must be maximized: the information transfer rate must be high, with little erroneous information (false alarms/detects); the information must be accurate and precise (robust high resolution estimates of range/angular/velocity for probing and navigation systems); and the extracted information should be hierarchically accessible (low resolution through high resolution access). All of these features should be simultaneously achieved.
Prior apparatuses and methods have tried to solve these problems and realize these desirable features, but without complete success. Typically, prior art modems or modulator/demodulators have embedded/extracted the desired information onto/from the amplitude, frequency, time, codes, and/or phase of a “base signal,” which is a sinusoidal or tonal signal. These apparatuses and/or methods have used a variety of well known modulation/demodulation techniques, including time-differential modulation, coherence multiplexing, transmitted reference communications, wavelet transform encoding/decoding, and ultra-wideband communications.
One example of a prior system which uses one or more of the techniques described above is disclosed in U.S. Pat. No. 4,065,718 to Attwood which is herein incorporated by reference. This prior system uses time differential modulation. One of the most significant problems with this system is its sensitivity to multipath degradations and its ability to be detected and exploited by eavesdroppers. The applied time-delay offset can be easily “realized” by the propagating environment (as a multipath) causing a false detection. Additionally, this modulation is easily detected by an eavesdropping listener (with a delay element in their processor). Furthermore, when this prior art modulator/demodulator is employed for probing or imaging as in radar, navigation, sonar and/or identification-friend-or-foe (IFF), the range/angle/velocity resolution performance is very poor. As a result, the problems of non-robustness, insecurity and poor performance limit the practical utility of this system.
Another example of prior apparatuses and/or methods which attempt to solve the problems discussed earlier are disclosed in U.S. Pat. Nos. 5,691,832, 4,860,279, 4,882,775, and 4,956,834 which are all herein incorporated by reference. These prior systems utilize optical modulation. More specifically, these systems utilize a single base signal and create a transmit signal by adding the original signal to a time-delayed version of the original signal and transmitting that composite signal pair. There is no no time-scale offset applied by this system. A significant shortfall of these systems is that if the propagating environment creates multipaths that are also on the order of the applied time-delay, performance can be significantly degraded due to false detections. Additionally, any eavesdropping listener or intercept receiver can easily detect the presence of this signal with a simple autocorrelative receiver. Further, the spatial resolution of a sum of delay-offset signals is proportional to the spatial extent of the signal (the duration of the signal multiplied by the speed of propagation in that medium) and this spatial/range resolution is very poor for long duration signals. Even further this signal has almost no relative velocity sensitivity and, as a result, velocity can not be used as a discriminator.
Another example of prior apparatuses and/or methods which attempts to solve the problems discussed earlier are disclosed in U.S. Pat. Nos. 5,761,238, 4,862,478, 5,774,493, and 5,774,493 which are all herein incorporated by reference. These prior systems utilize a transmitted reference and the code-division multiple-access (“CDMA”) modulation technique. In these systems, a reference signal is transmitted in a dedicated channel (analogous to a pilot signal). The transmitted reference signal is received and recovered as best as possible. The remainder of the received signal is demodulated by this estimated reference signal. A major limitation of CDMA systems is that reliably estimating the reference signal to high fidelity is very hard to do practically and suffers when the reference signal recreation is less than perfect. Obviously, any propagating environment that severely degrades the signal-of-interest will also severely degrade a transmitted reference signal. Simple linear correlation to recover the transmitted reference will not account for frequency dispersive, time-varying or non-linear degrading phenomenon. CDMA configurations with embedded pilot signals and RAKE receivers have the same limitations (low cost implementations are non-linear, frequency dispersive and time-varying). For all of these techniques, the receiver requires detailed knowledge of the reference and/or pilot channel (the code must be known). Further, stringent synchronization with the reference signal must be achieved to decode the embedded information; most CDMA receivers require extensive processing to synchronize the “code signal” with the received signal.
Yet another example of prior apparatuses and/or methods which attempt to solve the problems discussed earlier are disclosed in U.S. Pat. Nos. 5,729,750, 5,233,629, 5,963,581, 5,570,351, 5,625,642, and 5,561,431 which are all herein incorporated by reference. These prior systems utilize new modulations, such as orthogonal CDMA-multi-carrier, ultra-wideband, and wavelet. The inherent disadvantage of these systems is that they employ highly structured, and very limited “base signals” that are really just filter coefficients. None of these systems transmits signal pairs and modulate the relative offsets between those pairs. Only one base signal is used in a wavelet transform technique and it is termed the “mother wavelet” or “analyzing wavelet.” As a result, the sensitivity of these modulations follows that of CDMA—the analyzing wavelet must be known to extract the embedded information and any significant degradations of this reference signal will defeat information extraction. Additionally, the demodulators are not auto-correlative, they are matched filters with filter coefficients that replicate the “code signal” and require linear, dispersionless and intensive processing.
Yet another example of a prior system which attempts to solve the problems discussed earlier is disclosed in U.S. Pat. No. 5,990,823 which is herein incorporated by reference. This prior system discloses a wavelet-based radar. As with most wavelet-based sensing, the transmitted signal acts as the reference signal and the received signal is correlated against time-scaled and time-delayed versions of the transmitted signal (the time-scale and time-delay are induced by relative motion and the range, respectively, to scatterers in the environment). The problem with this prior system is that the velocity resolution is controlled by the time-scale resolution and is normalized by the speed of light. To estimate velocities less than 500 mph requires unreasonably long, coherently processed receive signals (multiple seconds).
Accordingly, as illustrated in the discussion above prior apparatuses and/or methods fail to simultaneously realize all or even most of the aforementioned desirable features or goals. For example, to be secure most active systems must use low power transmissions and signals that possess both long durations and high bandwidths (high time-bandwidth product signals). However, although these qualities are desirable for security, they are undesirable for robust detection, low cost implementations and efficiency. A spread spectrum modulation scheme requires expensive linear and matched components, high rate processing, almost perfect synchronization, minimum dispersion and detailed knowledge of the signal structure at both ends of the modem. Thus, design tradeoffs occur that sacrifice either security, performance and/or costs. Each of the desirable features usually have conflicting requirements leading to trade off performance in one feature for lack of performance in another feature. None of the prior apparatuses and/or methods simultaneously solves these tradeoff problems.
Additionally, prior systems that transmit the sum of two offset copies of a signal suffer from poor range/angle/velocity resolution; potential exploitability; sensitivity due to multipaths; and very low information transfer/extraction rates. The prior art that employs high time-bandwidth product signals or “coded signals,” suffer from very high processing requirements; expensive precision components and element arrays; extreme sensitivity to frequency dispersion, relative motion, and non-linearities; and the requirement that all users of the signals “know the codes.” When a propagating environment or sensor system significantly degrades the coherence of the received signal relative to the transmitted signal, the performance of these high time-bandwidth product signal techniques degrade rapidly; the “matched filter processor” is no longer matched.